1. Field of the Invention
This invention relates to a method of producing a magnetic material for use in a magnetic recording medium and, in particular, to a new metal powder magnetic material and a method for the production thereof.
2. Description of the Prior Art
Conventionally, powdered ferromagnetic materials such as .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co containing .gamma.-Fe.sub.2 O.sub.3, Co containing Fe.sub.3 O.sub.4, CrO.sub.2, etc., have been used for magnetic recording media. Such materials, however, are not completely satisfactory for recording signals with short wave-lengths (about 10 microns or less). In other words, their magnetic properties of coercive force (H.sub.c) and maximum residual magnetic flux density (B.sub.r) are insufficient. Recently, various ferromagnetic powder materials suitable for high density recording have been extensively developed. One group of such materials is a finely divided ferromagnetic metal, comprising pure metal, a metal alloy or an intermetallic compound. Suitable metals include Fe, Co, and Ni, to which, depending on the requirements, Cr, Mn, rare earth metals, Zn, etc., are added.
The following methods of producing ferromagnetic materials are known.
(I) Hydrolysis of an organic acid salt of a ferromagnetic metal, followed by reduction with a reducing gas, as is described in, for example, Japanese Patent Publication Nos. 11412/1961 and 38417/1972.
(II) Reduction of needle-shaped iron oxyhydroxide, needle-shaped iron oxyhydroxide containing a foreign metal, or needle-shaped iron oxide obtained from these oxy-hydroxides, as is disclosed in Japanese Patent Publication Nos. 3862/1960 and 39477/1972, or in British Patent No. 1,192,167.
(III) Vaporization of a ferromagnetic material in an atmosphere of an inert gas, as is described in Japanese Patent Publication No. 27718/1972, or "Ohyo Butsuri" (Applied Physics) Vol. 40, No. 1, p. 110 (19671).
(IV) Decomposition of a metal carbonyl compound, as is described in U.S. Pat. Nos. 2,983,997 and 3,228,882.
(V) Electrodeposition of a ferromagnetic metal onto a mercury cathode, followed by separation of the metal from the mercury, as is disclosed in Japanese Patent Publication Nos. 15525/1964 and 8123/1965.
(VI) Reduction of a salt of a ferromagnetic metal in solution, as is described in Japanese Patent Publication Nos. 20520/1963, 26555/1963, 20116/1968 and 41718/1972, U.S. Pat. Nos. 3,663,318 and 3,661,556, and German Patent OLS 2,132,430.
The present invention relates to method (VI) above and especially to a method which employs a borohydride compound or a derivative thereof as a reducing agent.
Conventional methods of reducing a ferromagnetic metal salt in solution with a borohydride compound or a derivative thereof, have the following drawbacks. Firstly, the metal powder thus prepared, especially when the powder contains Fe, is sensitive to moisture so that it is gradually oxidized even when kept at room temperature conditions. In an extreme case, it has been proved that the ferromagnetic powder is finally converted into a non-magnetic material. Secondly, because of the high surface activity of the powder prepared by this method, the treatment of the powder during production has been quite difficult from an industrial standpoint. That is, a danger of fire exists when such powder is treated in the air; moreover, this active powder tends to decompose or degrade a binder material when blended therewith, thus causing poor dispersion or difficulties in coating.